The invention resides in an actuator for an optical-mechanical scanner and a method of using the actuator.
Optical-mechanical scanners are used in reading devices for the identification of bar codes (barcode scanners). The operation of such scanners is based essentially on the photo-technical scanning of a surface area to be identified with a periodically deflected light beam which is focused onto the surface area for achieving a resolution as required for a particular application. The time-dependent pattern of the reflected light is then received by a photo-electric receiver and converted into electrical signals. From the time-dependent pattern of these signals, the surface area to be identified is reconstructed.
Optical-mechanical scanners therefore comprise a unit for the deflection of the beam, which is based on different operational principles. Important for the arrangement is the intended application. If a scanner is intended for scanning text or even color images with high resolution, the beam deflection occurs generally by way of relatively slow mechanical devices over a large area. In contrast, scanners which are used for example in cash registers for the identification of bar codes must be able to detect the coded information rapidly, that is, in a single scanning procedure although with relatively low, but still sufficient lateral resolution. In this application, there is no need to involve the whole of a surface area by way of several staggered scanning procedures. However, in all devices, for the deflection of the beams, the light beam must be moved over the object being scanned in a uniform manner.
At the end of the description literature [1] to [6] is listed. In [1] a device for the beam deflection is described which is considered to be the technically and economically best solution and which is used in almost all optical-mechanical scanners suitable for the photo-technical scanning of a barcode. It consists of a rotating polygonal mirror with several planar mirror surfaces, onto which a light beam from a laser diode is directed sequentially with the direction of rotation of the rotating mirror surfaces. The laser beam is consequently reflected onto the barcode with twice the circumferential speed and the process is repeated with each mirror surface area reaching the laser beam. As scanning frequency [1] a range of 200 to 800 Hz and as usable scan angle, an angle of 58° are indicated.
Another device of this type is disclosed in [2].
In connection with handheld scanners for scanning bar codes, but also for other scanners, particularly mobile text scanners, a miniaturization of the beam deflection device is desired.
Devices for the beam deflection using rotating polygonal mirrors however do not appear to be very suitable herefo because of the necessary drive means. New concepts circumventing this limitation are based on devices with actuators utilizing electromagnetic, magneto-restrictive or electrostatic effects for the beam deflection.
In this connection [3] discloses a device for the beam deflection by means of a magneto-strictive actuator which carries a mirror surface and vibrates in a resonance mode. The magnetic field which is generated by two Helmholtz coils acts herein on a cantilever bimorph resonator. As scanning frequency, a range of 10 to 50 kHz with a usable scanning angle of 24° is indicated.
In [4] and [5] alternative arrangements for the beam deflection by micro-actuators utilizing electrostatic [4] or electromagnetic [5] forces are described. Also those actuators act like spring-mass systems whose amplitude-frequency characteristics are characterized by a high frequency dependdecy, by different oscillation modes and by secondary resonances.
It is the object of the present invention to provide an actuator for the beam deflection in an optical-mechanical scanner and a method of using such an actuator wherein particularly the disadvantages caused by spring-mass systems are avoided to a large extent.